Evaluating triggers for river capture along the Sutlej River, western Himalaya

Abstract:

Drainage capture is a ubiquitous phenomenon in the Himalayan orogen, as reflected in numerous barbed drainages and asymmetric basins that extend across the orogen and into southern Tibet. Field evidence and a series of migrating knickpoints in the Zhada Basin of southern Tibet suggest late Pleistocene capture of the basin by the trans-Himalayan Sutlej River. We use a numerical incision model to constrain the incision magnitude and temporal evolution in response to this event. Modeling results, in conjunction with field observations, topographic analyses, and low-T thermochronometry, are used to evaluate two different proposed triggers for drainage capture. First, we examine a potentially Quaternary-active South Tibetan Detachment System (STDS) as a driver of base level fall and subsequent divide migration. While activity along the STDS in the western Himalaya is believed to have ceased in the Miocene, apatite (U-Th)/H­e ages along the Sutlej River valley suggest differ­ential exhumation across the STDS during the Quaternary. Second, we evaluate the possibility that an earlier capture of a large downstream tributary to the Sutlej (Spiti River) triggered capture of the Zhada Basin. The findings of this work have implications for similar capture events by other trans-Himalyan rivers (e.g. Arun River, Kali Gandaki, Trishuli River). This work is vital to understand how orogenic systems evolve and whether Himalayan drainage reorganization is primarily a product of internal dynamics or tectonic ­forcings.